Analog-to-digital converters (ADCs) may include loop filters that may be tunable low-pass or band-pass filters. FIG. 1 illustrates an exemplary Sigma-Delta Modulation (SDM) type of ADC 100 as commonly used in the art. As shown in FIG. 1, the SDM ADC may include a forward signal path and a feedback signal path. The forward signal path may include a loop filter 20, a resonator 24, a summer 26, and a flash ADC 28. The loop filter 20 may further include three additional resonators 14, 16, 18. The feedback signal path may include an element selection logic 36, digital-to-analog converters 30, 32, 34 through which the digital output signal may be coupled to the forward signal path respectively through adders 12, 22 and summer 26.
Commonly, the loop filter 20 may have a center frequency f0 that may be tunable in a frequency range as wide as from DC to 1 GHz. To achieve the wide range of tunable center frequencies, resonators 14, 16, 18 inside the loop filter 20 may be designed to be widely tunable. The first resonator 14 may be switched from an active RC resonator for f0<200 MHz to an LC resonator for f0≧200 MHz. The second resonator 16 and the third resonator 18 may be active RC resonators with both the resistor R and capacitor C being tunable so that the center frequency f0 of the overall loop filter may be in a range from approximately DC to approximately 1 GHz.
A resonator may be characterized by a quality factor (or Q factor) which may measure how under-damped the resonator may be. The Q factor also may be measured in terms of the bandwidth of the resonator relative to the resonance frequency of the resonator. In general, a resonator having a higher Q factor may be more efficient and therefore more desirable. For example, for a resonator having a resonance frequency of approximately 1 GHz, it may be desirable that the Q factor is greater than 30.
The resonance frequency of a tunable active RC resonator may be tuned through tunable resistors and/or tunable capacitors. Commonly, the tunable resistors and tunable capacitors may be implemented with digitally-controlled MOSFET switches. These MOSFET components may introduce parasitic components having parasitic capacitance or parasitic resistance into the active RC resonator circuitry. These parasitic components may degrade the Q factor of the active RC resonator. Therefore, there is a need for systems and methods that may counter the Q factor degradation caused by these parasitic components and thus enhance the Q factor of the active RC resonator.